Various apparatuses and methods for determining coordinates of measurement objects, for example turbine blades, vehicle body sheets, seals or printed circuit boards, are known from the related art. In particular, the determination of an orientation of edges of the measurement object is important, for example, because reference coordinate systems that are employed are frequently defined in relation to edges.
For measuring edges, for example, imaging methods are known in which the focus is traversed in increments (“infinite focus”) and the orientation of the edges is determined by way of edge detection. However, such methods may require a long measurement time.
Also known are confocal and confocally chromatic distance sensors. In sensors of this type, depth determination is performed by evaluating an intensity distribution along a height coordinate z. By way of example, confocally chromatic sensors permit measurement of the height coordinate z with a single image. In principle, in the case of such sensors, use is made of measurement objective lenses having chromatic aberration, which focus portions of a light beam with different wavelengths in different focal planes along an optical axis of the sensor. Each wavelength of the light beam can thus be assigned a height coordinate. If a focus of a wavelength is located on a surface of the measurement object, an intensity maximum can be measured for the wavelength on a sensor element of the confocally chromatic sensor and the depth can thus be determined.
When using confocally chromatic sensors for non-contact measurement of a three-dimensional measurement object, measurement artefacts can occur at edges that make exact localization of the edge difficult. In confocally chromatic sensors, a longitudinal chromatic aberration is introduced, which can have the effect that light is reflected and/or scattered by the measurement object and detected by a sensor even if the edge is not located on an optical axis of the confocally chromatic sensor and/or the edge no longer shadows the optical axis. The sensor can detect, for example, light having a shorter or longer wavelength as compared to a confocal wavelength with a longitudinal chromatic aberration. For these non-confocal wavelengths, it is also possible for a spectral peak to be determined by the confocally chromatic sensor even if the optical axis is no longer covered by the edge. In addition, a height coordinate which is thus determined can deviate from an actual height coordinate.